FIG. 1 on the appended drawing shows a known subscriber line interface circuit (SLIC) 1 comprising two current amplifiers 2, 3 for supplying a line current IL on their respective output terminal to a tip wire A and a ring wire B, respectively, of a two-wire transmission line connected to a load RL, e.g. a telephone set, resulting in a line voltage VL across the load RL.
In FIG. 1, the current amplifier 2 is supposed to have a gain of +g, while the current amplifier 3 is supposed to have a gain of −g.
The line current supplied by the respective current amplifier 2, 3 to the wires A and B is composed of currents controlled by a DC feed unit 4, an AC transmission unit 5, and a longitudinal suppression unit 6.
In a manner known per se, the DC feed unit 4 supplies DC currents IDC to the current amplifiers 2, 3 in accordance with a predetermined DC feed characteristic such that a maximum line current IL is supplied when the line voltage VL=0, and a line current IL=0 is supplied when the line voltage VL is at its maximum. The DC currents IDC are multiplied with the gains +g and −g, respectively, to form so-called transversal line currents in the wires A and B, which determine the voltage VL across the load RL, i.e. the line voltage,
Transversal currents are by definition, currents in the wires A and B that are of the same value but opposite polarity in the respective wire.
Also, in a manner known per se, the AC transmission unit 5 supplies AC currents IAC to the current amplifiers 2, 3 in response to AC signals, e.g. outgoing speech signals received from a D/A converter (not shown) as indicated by an arrow in FIG. 1. The AC currents IAC are also multiplied with the gains +g and −g, respectively, to form so-called transversal AC currents in the wires A and B, i.e. AC currents that are of the same value but of opposite polarity in the respective wire. In this connection, it should be mentioned that the transversal AC currents are the only signals that can be converted to audible signals in a loudspeaker at the load RL.
The current amplifier 2 is supposed to be connected with its voltage supply terminal to ground (not shown), and the current amplifier 3 is supposed to be connected with its voltage supply terminal to a negative voltage source (not shown), Thus, the voltage VL across the load RL has to be generated between ground and negative voltage. To avoid that the current amplifiers 2, 3 become saturated to ground and the negative voltage, respectively, the longitudinal suppression unit 6 controls the output terminals of the current amplifiers 2 and 3 by means of currents +ILNG and −ILNG, respectively, to a predetermined DC voltage value.
In some applications, the output terminal of the current amplifier 2, i.e., the A wire, is controlled such that its desired value is set to a certain voltage below ground. Then, the voltage on the output terminal of the current amplifier 3, i.e. the B wire, is set by the voltage on the A wire minus the voltage VL across the load RL.
In other applications, the mid-point between ground and the negative voltage is detected and the current amplifiers 2, 3 are controlled such that the desired voltage value across the load RL is symmetric relative to this mid-point.
In all these applications, the longitudinal suppression unit 6, in a manner known per se, detects an actual value of the above voltages and controls, by means of the currents +ILNG and −ILNG, the output terminals of the current amplifiers 2, 3, i.e. the A wire and the B wire, to the desired value.
Hereby, any so-called longitudinal currents in the wires A and B will be suppressed as will be described more in detail below with reference to appended FIG. 3.
Longitudinal currents are by definition, externally originating disturbance currents in the wires A and B that are of the same value and the same polarity in both wires.
Incoming AC signals on the transmission line A, B, e.g. incoming speech signals, are detected by a differential amplifier 7 connected with its two input terminals to the respective wire A, B.
The output terminal of the differential amplifier 7 is connected to an A/D converter (not shown) as indicated by an arrow in FIG. 1.
Today, SLICs are normally manufactured on silicon dies.
To reduce manufacturing costs, there is a desire to reduce the silicon area as much as possible. This can be accomplished e.g. by designing simpler SLICs. Hereby, less power will also be consumed.
Thus, there is a desire to miniaturize the SLIC dies as much as possible. Also, there is a desire to reduce the power consumption of the SLICs on such dies.